Flexible laminate board, process for manufacture of the board, and flexible print wiring board

ABSTRACT

A process for production of a flexible laminated sheet having one or more laminated bodies each provided with a metal foil formed on one side of a resin film. The process includes coating a varnish containing a polyamic acid and a solvent onto the metal foil, holding the coated film, drying in which at least a portion of the solvent in the varnish is removed to form a layer composed of a resin composition, and forming the resin in which the layer composed of the resin composition is heated to form a resin film containing a polyimide resin. The conditions for each step from the coating up to the resin film-forming are adjusted based on a target for the content of metal elements in the resin film.

This application is a Continuation application of Application Ser. No.12/091,519, having a filing date under 35 USC 371 of Nov. 21, 2008, thecontents of which are incorporated herein by reference in theirentirety. Ser. No. 12/091,519 is a National Stage Application, filedunder 35 USC 371, of International (PCT) Application No.PCT/JP2006/320851, filed Oct. 19, 2006.

TECHNICAL FIELD

The present invention relates to a flexible laminated sheet that can beused for manufacture of flexible printed circuit boards, and to aprocess for its production. The invention further relates to a flexibleprinted circuit board.

BACKGROUND ART

A flexible printed circuit board is a flexible wiring board with aconductor pattern formed on the surface of an insulating resin film.Flexible printed circuit boards have become common in recent years asmeans of achieving increased miniaturization and higher density inelectronic devices. Most flexible printed circuit boards employ aromaticpolyimides as resin films.

Flexible printed circuit boards employing aromatic polyimides haveconventionally been manufactured by a process in which, generally, acopper foil is bonded to a polyimide film as the insulating layer usingan adhesive such as an epoxy resin or acrylic resin. With flexibleprinted circuit boards obtained by this process, the level of propertiessuch as heat resistance, chemical resistance, flame retardance,electrical characteristics and adhesiveness depend on the properties ofthe adhesive used, and therefore it has not been possible tosatisfactorily exhibit the excellent properties of aromatic polyimides.

Methods of heat sealing polyimide films to metal foils usingthermoplastic polyimides as adhesives have therefore been proposed(Patent documents 1-3). There have also been proposed methods of directcast coating of a metal foil such as a copper foil with a solution of apolyamic acid (polyimide precursor) having a thermal expansioncoefficient equivalent to that of the metal foil, and removing thesolvent to produce a high molecular weight product (hereinafter referredto as direct coating methods) (Patent documents 4, 5). There are alsoknown methods of forming metal layers by vapor deposition or sputteringonto polyimide films (Patent document 6).

-   [Patent document 1] Japanese Unexamined Patent Publication HEI No.    3-104185-   [Patent document 2] Japanese Unexamined Patent Publication No.    2005-44880-   [Patent document 3] Japanese Unexamined Patent Publication No.    2005-96251-   [Patent document 4] Japanese Unexamined Patent Publication SHO No.    58-190093-   [Patent document 5] Japanese Unexamined Patent Publication SHO No.    63-69634-   [Patent document 6] Japanese Patent Publication No. 3447070

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

With flexible printed circuit boards obtained by conventional productionprocesses including those described in Patent documents 1-6, however, ithas been difficult to sufficiently reduce the permittivities of thepolyimide films. Consequently, further improvement is desired in termsof the characteristics, and especially dielectric characteristics, ofthe flexible printed circuit boards.

It is therefore an object of the present invention to provide a processfor production of a flexible laminated sheet that can be used tomanufacture flexible printed circuit boards that exhibit satisfactorydielectric characteristics, by comprising a resin film containing apolyimide resin with sufficiently reduced permittivity.

The flexible printed circuit boards obtained by conventional productionprocesses have not been adequately resistant to wire breakage caused bypeeling of wiring under repeated flexural stress or thermal history. Inother words, further improvements in terms of flexible printed circuitboard reliability are desired.

For example, the aforementioned flexible printed circuit boards of theprior art which employ thermoplastic polyimides as adhesives do notalways exhibit adequate heat resistance by the thermoplastic polyimides,and therefore the resistance to thermal history has been insufficient.The high molding temperature required for heat sealing has also led toproblems of increasing complexity of the production equipment. Theflexible printed circuit boards obtained by conventional direct coatingmethods have also often been insufficient from the standpoint ofreliability.

On the other hand, sputtering production processes require specialequipment for sputtering, and the problem of production step complexityarises when plating and high-temperature heat treatment steps arenecessary.

It is therefore another object of the invention to provide a process forproduction of a flexible laminated sheet that allows production offlexible printed circuit boards with sufficiently high reliability usingsimple steps.

Means for Solving the Problems

The present inventors have studied the cause of the high permittivity ofpolyimide films in flexible laminated sheets, from the viewpoint of theelements in the films. As a result, it was discovered that the polyimidefilms contain metal elements that should be absent from the startingmaterials for polyimide films. Upon further research, the presentinventors found that the metal elements in polyimide films are the samespecies as the component elements of the adjacent metal foil, and theirconcentration distribution in the thickness direction decreases from themetal foil side of the polyimide film toward the opposite side.

The present inventors therefore surmised that the content of the metalelements in the polyimide film increases because the metal elements movefrom the adjacent metal foil into the polyimide film of the flexiblelaminated sheet (a phenomenon known as “migration”), thus causing anincrease in permittivity. As a result of yet further detailedinvestigation on flexible laminated sheet production processes with thegoal of preventing such migration, the present invention was completed.

One aspect of the invention is a process for production of a flexiblelaminated sheet having one or more laminated bodies each provided with ametal foil formed on one side of a resin film, the process comprising acoating step in which a varnish containing a polyamic acid and a solventis coated onto the metal foil to form a coated film, a holding step inwhich the coated film formed on the metal foil is held, a drying step inwhich at least a portion of the solvent in the varnish is removed toform a layer composed of a resin composition, and a resin film-formingstep in which the layer composed of the resin composition is heated toform a resin film containing a polyimide resin, wherein the conditionsfor each step after the coating step up to the resin film-forming stepare adjusted based on a target for the content of metal elements in theresin film.

According to the invention it is possible to sufficiently reduce thepermittivity of the resin film containing the polyimide resin which isformed on the flexible laminated sheet, so that a flexible printedcircuit board fabricated from the flexible laminated sheet can exhibitsatisfactory dielectric characteristics. The present inventors believethe reasons for this effect to be the following. Other factors, however,may be involved.

For conventional fabrication of flexible laminated sheets with polyimidefilms, the metal foil after coating of the varnish containing thepolyamic acid and solvent is usually stored for a certain period of time(for example, 1-2 days) in air at room temperature without active dryingof the varnish, from the viewpoint of allowing more flexibility in theproduction steps. In most cases, this is followed by removal of thesolvent in the varnish to form a layer composed of the resincomposition, but at times the entire solvent is removed while at othertimes only a portion of the solvent is removed. In addition, differenttemperatures and atmospheres are used for the solvent removal. Theconditions for curing of the layer composed of the resin composition(for example, the temperature and atmosphere) are adjusted according tothe type of polyimide. However, migration of metal elements from themetal foil into the polyimide film is believed to occur because of theacidic polyamic acid in the varnish coated on the metal foil.Specifically, it is conjectured that the polyamic acid dissolves themetal foil and promotes migration of the metal elements in the metalfoil into the varnish, thus resulting in inclusion of metal elementsinto the polyimide film obtained from the varnish.

However the conditions, including the temperature, atmosphere and timefor storage, the temperature and atmosphere for removal of the solventin the varnish, the degree of removal of the varnish and the atmospherefor curing of the resin composition, have not been considered from theviewpoint of the content of metal elements in the polyimide film. Thecontent of metal elements in the polyimide film is therefore affected bythe polyamic acid, such that it has not been possible to control thepermittivity of the polyimide film sufficiently as desired.

According to the production process of the invention, however, thevarious conditions for the step after coating of the varnish on themetal foil and for the steps prior to formation of the resin film, suchas the holding step and/or drying step, are adjusted based on a targetfor the content of metal elements in the resin film. This allows thepermittivity of the resin film to be sufficiently controlled. A resinfilm with an adequately reduced permittivity is thus obtained,permitting fabrication of a flexible printed circuit board that exhibitssatisfactory characteristics.

According to the invention, the conditions for each of the steps arepreferably adjusted so that the content of metal elements in the resinfilm is no greater than 5 wt %. Investigation by the present inventorshas suggested that a content of no greater than 5 wt % for metalelements in the resin film tends to result in a more sufficientpermittivity for practical use.

According to the invention, the layer composed of the resin compositionis preferably heated in a reducing atmosphere during the resinfilm-forming step. This allows oxidation of the metal foil to be moreeffectively prevented, for improved adhesiveness between the metal foiland resin film in the flexible laminated sheet.

Another aspect of the invention is a process for production of aflexible laminated sheet having one or more laminated bodies eachprovided with a metal foil formed on one side of a resin film, theprocess comprising a coating step in which a varnish containing asolvent and either a polyimide resin or its precursor is coated onto themetal foil, a drying step in which the solvent in the varnish is removedto a proportion of 1-60 wt % of the total, and a resin film-forming stepin which the resin composition layer is heated to 250-550° C. under areducing atmosphere to form a resin film containing a polyimide resin.

According to this production process of the invention, the varnish isdried until the solvent proportion is within the aforementionedspecified range and is then heat treated at a temperature within theaforementioned specified range to produce a polyimide resin, andtherefore a flexible laminated sheet that can yield flexible printedcircuit boards with sufficiently high reliability can be produced by asimple process.

In the production processes described above, the reducing atmosphere ispreferably one formed by a mixed gas composed of nitrogen gas withhydrogen gas at between 0.1 vol % and 4 vol % of the total. This willmore reliably prevent reduction in reliability by oxidation of the metalfoil, while also allowing fabrication of a flexible laminated sheet viasafer steps.

In the drying step, the varnish is preferably heated to 100-170° C. forremoval of the solvent. The metal foil is preferably a copper foil.

The invention provides a flexible laminated sheet comprising one or morelaminated bodies each having a metal foil formed on one side of a resinfilm, wherein the content of metal elements in the resin film is nogreater than 5 wt %. The metal foil-clad flexible laminated sheet isobtained by the production processes of the invention described above.The flexible laminated sheet can be used to obtain flexible printedcircuit boards exhibiting sufficiently satisfactory dielectriccharacteristics.

A metal foil-clad flexible laminated sheet of the invention is aflexible laminated sheet obtained by the production processes of theinvention described above. The flexible laminated sheet can be used toobtain flexible printed circuit boards with sufficiently highreliability.

The relative permittivity of the resin film in the flexible laminatedsheet of the invention is preferably no greater than 3.3 at 5 GHz. Thiscan further increase the reliability of the flexible printed circuitboard. The resin film in the flexible laminated sheet of the inventionpreferably has a relative permittivity of no greater than 3.3 at 5 GHz,a thermal expansion coefficient of no greater than 25 ppm/° C. and apeel strength of the resin film from the resin film is at least 1.2kN/m. This can further increase the reliability of the flexible printedcircuit board.

The flexible printed circuit board of the invention has a conductorpattern formed by removing a portion of the metal foil on the flexiblelaminated sheet of the invention. Alternatively, the flexible printedcircuit board of the invention may be a one obtainable by removing themetal foil and forming a conductor pattern on the exposed resin film.Such flexible printed circuit boards have sufficiently high reliabilitysince they are produced using a flexible laminated sheet of theinvention as described above.

Effect of the Invention

According to the invention there is provided a process for production ofa flexible laminated sheet that can be used to fabricate flexibleprinted circuit boards that exhibit satisfactory dielectriccharacteristics, by comprising a resin film containing a polyimide resinwith sufficiently reduced permittivity. Also, the process for productionof a flexible laminated sheet according to the invention can be used toproduce a flexible laminated sheet that allows manufacturing of flexibleprinted circuit boards with sufficiently high reliability by simplesteps.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of a flexiblelaminated sheet according to the invention.

FIG. 2 is a cross-sectional view of an embodiment of a flexiblelaminated sheet according to the invention.

FIG. 3 is a cross-sectional view of an embodiment of a flexiblelaminated sheet according to the invention.

FIG. 4 is a cross-sectional view of an embodiment of a flexible printedcircuit board according to the invention.

FIG. 5 is a cross-sectional view of an embodiment of a flexible printedcircuit board according to the invention.

EXPLANATION OF SYMBOLS

1 a, 1 b, 1 c: Flexible laminated sheets, 2 a, 2 b: flexible printedcircuit boards, 10: laminated body, 11: resin film, 12: metal foil, 15:adhesive layer, 20: conductive pattern.

BEST MODES FOR CARRYING OUT THE INVENTION

Preferred embodiments of the invention will now be described in detail.However, the present invention is not limited to the embodimentsdescribed below.

FIG. 1 is a cross-sectional view of an embodiment of a flexiblelaminated sheet according to the invention. The flexible laminated sheetla shown in FIG. 1 comprises a laminated body 10 formed by bonding ametal foil 12 onto one side of a resin film 11. The thickness of theresin film 11 will normally be about 1-100 μm.

The content of metal elements in the resin film 11 is preferably nogreater than 5 wt %. If the content exceeds 5 wt %, the permittivity ofthe resin film 11 will tend to be too high for practical use. Inparticular, a metal element content of greater than 10 wt % will notablyimpair the electrical characteristics including the dielectriccharacteristic of the flexible printed circuit board. Therefore, fromthe viewpoint of more reliably maintaining the electricalcharacteristics of the flexible printed circuit board, the metal elementcontent is preferably no greater than 5 wt %. This tendency is morepronounced when the metal foil 12 is a copper foil. While a lower metalelement content is preferred, the lower limit will usually be about 2 wt%.

The metal element content of the resin film 11 can be measured by X-rayPhotoelectron Spectroscopy (XPS). The measurements referred tothroughout the present specification are all preceded by removal of themetal foil 12 from the resin film 11. Then, argon etching is carried outfrom the side of the resin film 11 in contact with the metal foil, to aprescribed depth in the direction of thickness. Next, an ESCA5400 X-rayphotoelectron spectrometer (trade name of Ulvac-Phi, Inc.) is used formeasurement of the quantity of metal elements in the surface exposed byetching. Argon etching is then continued to the next prescribed depth,and the metal element quantity is measured in the same manner. Theprocedure is repeated until the metal element quantity reaches thedetection limit. Finally, the metal element quantity with respect to thetotal resin film 11 is calculated from the metal element quantity ateach depth, and the content is calculated therefrom.

The relative permittivity of the resin film 11 at 5 GHz is preferably nogreater than 3.3. The relative permittivity is an index of theinsulating property of the resin film, and in cases where the pitchbetween copper wirings is narrowed or the interlayer thickness isreduced as means for achieving miniaturization and high density ofelectronic devices, it is preferred for the relative permittivity of theresin film to be a small value. Electronic devices must be operated athigh frequency, and a low relative permittivity in the gigahertz band isespecially preferred.

The relative permittivity of the resin film 11 can be easily measured bya method using a resonant cavity perturbation complex permittivityevaluator (hereinafter referred to as “cavity resonator”). When therelative permittivity is measured using a cavity resonator, such as a“CP511” (trade name of Kantoh Electronics Application and DevelopmentInc.), measurement may be performed with three test resin films having athickness of 0.6 mm, a width of 1.8 mm and a length of 80 mm, and themean value recorded as the relative permittivity. In cases where thethickness of the test films is insufficient, a plurality of resin filmsmay be stacked to ensure that the prescribed thickness is obtained.

The coefficient of linear thermal expansion of the resin film 11 ispreferably no greater than 25 ppm/° C. and more preferably 10-25 ppm/°C. The coefficient of linear thermal expansion is an index of theheat-dependent elongation percentage of the material, and when two ormore different materials are attached, a smaller difference incoefficients of linear thermal expansion of the materials is preferredfrom the viewpoint of reliability. The coefficient of linear thermalexpansion of the metal foil will generally be 10-25 ppm/° C. (forexample, 10 ppm/° C. for stainless steel, 20 ppm/° C. for copper alloysor 22 ppm/° C. for aluminum alloys). When resin films with coefficientsof linear thermal expansion exceeding 25 ppm/° C. are attached to themetal foils, warping tends to occur upon heating during the attachmentor after the attachment, and wiring breakage or molding defects becomemore likely, thus reducing the reliability.

The coefficient of linear thermal expansion may be conveniently measuredby a TMA method. For measurement of the coefficient of linear thermalexpansion by TMA, for example, a test piece with a thickness of 0.025mm, a width of 13 mm and a length of 15 mm may be raised to atemperature of from 50° C. to 300° C. at 10° C./min using a “TMA2940”(trade name of TA Instruments) under a load of 0.5 gf, and then cooledto room temperature and again raised in temperature from 50° C. to 350°C. at 10° C./min under a load of 0.5 gf, at which time the coefficientof linear thermal expansion in the range of 50° C.-250° C. may becalculated to determine the coefficient of linear thermal expansion.

The peel strength of the metal foil from the resin film is preferably atleast 1.2 kN/m. The peel strength of the metal foil from the resin film,i.e. the adhesive force, is related to the reliability when wiring isformed by etching or the like. For improved reliability, it is desirablethat no peeling or wire breakage occur under the stress of repeatedbending or with thermal history. The peel strength is the maximum valueof stress at which the metal foil of a test piece with a thickness of0.025 mm and a width of 10 mm peels from the main side of the resin filmat an angle of 90 degrees.

The present inventors measured metal element contents and relativepermittivities by the methods described above. However, values measuredby methods under different conditions including different apparatusesand different test piece shapes can be compared if compensation is made.

As the metal foil 12 there may be suitably used foils of copper,aluminum, iron, gold, silver, nickel palladium, chromium and molybdenumor their alloys. Copper foil is preferred among the above. In order toincrease the adhesive force with the resin film 11, the surface may bemechanically or chemically treated by chemical roughening, coronadischarge, sanding, plating or treatment with aluminum alcoholate,aluminum chelate or silane coupling agents.

The flexible laminated sheet 1 a is obtained by a production processcomprising a coating step in which a varnish containing a polyamic acidand a solvent is coated onto the metal foil 12 to form a coated film, aholding step in which the coated film formed on the metal foil is held,a drying step in which at least a portion of the solvent in the varnishis removed to form a layer composed of a resin composition (hereinafterreferred to as “resin composition layer”), and a resin film-forming stepin which the resin composition layer is heated to form a resin film 11containing a polyimide resin, wherein the conditions for each step afterthe coating step up to the resin film-forming step are adjusted based ona target for the content of metal elements in the resin film 11.

Alternatively, the flexible laminated sheet la may be produced by aproduction process comprising a coating step in which a varnishcontaining a solvent and either polyamic acid as a polyimide resinprecursor is coated onto a metal foil, a drying step in which thesolvent in the varnish is removed to a proportion of 1-60 wt % of thetotal, and a resin film-forming step in which the resin compositionlayer is heated to 250-550° C. under a reducing atmosphere to remove thesolvent remaining in the resin composition layer while forming apolyimide resin from the polyamic acid, to form a resin film 12containing the polyimide resin.

The varnish used in the coating step contains one or more polyamic acidsas polyimide resin precursors. The polyamic acid is converted to apolyimide resin by heating primarily in the resin film-forming step.

The concentration of the polyamic acid in the varnish is preferably 8-40wt %. The viscosity of the varnish is preferably 1-40 Pa-s and morepreferably 10-40 Pa·s. If the viscosity of the varnish is outside ofthis range, defects in appearance such as cissing may result uponcoating onto the metal foil, thus tending to lower the film thicknessprecision.

In the coating step, two or more different varnishes may be coated oneafter the other. In this case, a separate varnish may be coated onto thecoated film formed on the metal foil, and a varnish separate from thaton the lower layer may also be coated over the resin composition layerafter the drying step or over the resin film after the resinfilm-forming step.

The polyimide resin is a polymer with imide groups on the main chain,and for example, it contains polymer chains represented by the followinggeneral formula (1).

Polyamic acids contain amide and carboxyl groups and are precursors ofpolyimide resins. The amide groups and carboxyl groups of a polyamicacid react under heating to form imide groups, thus resulting inconversion to a polyimide resin. For example, the polyamic acid may be apolymer with a polymer chain represented by the following generalformula (2).

In formulas (1) and (2), R¹ represents a residue obtained by removing anamino group from a diamine, or a residue obtained by removing anisocyanato group from a diisocyanate, and R² represents a residueobtained by removing the carboxylic acid derivative portion of anaromatic tetracarboxylic acid derivative. The letter n represents aninteger of 1 or greater.

The polyamic acid may be synthesized by reacting a tetracarboxylic acidor its derivative with a diamine and/or diisocyanate.

An aromatic amine is preferred as the diamine. As specific examples ofaromatic amines there may be mentioned p-, m- or o-phenylenediamine,2,4-diaminotoluene, 2,5-diaminotoluene, 2,4-diaminoxylene,diaminodurene, 1,5-diaminonaphthalene, 2,6-diaminonaphthalene,benzidine, 4,4′-diaminoterphenyl, 4,4′″-diaminoquaterphenyl,4,4′-diaminodiphenylmethane, 1,2-bis(anilino)ethane,4,4′-diaminodiphenyl ether, diaminodiphenylsulfone,2,2-bis(p-aminophenyl)propane, 2,2-bis(p-aminophenyl)hexafluoropropane,2,6-diaminonaphthalene, 3,3-dimethylbenzidine,3,3′-dimethyl-4,4′-diaminodiphenyl ether, 3,3′-dimethyl-4,4′-diaminodiphenylmethane, diaminotoluene, diaminobenzotrifluoride,1,4-bis(p-aminophenoxy)benzene, 4,4′-bis(p-aminophenoxy)biphenyl,2,2′-bis[4-(p-aminophenoxy)phenyl]propane, diaminoanthraquinone,4,4′-bis(3-aminophenoxyphenyl)diphenylsulfone,1,3-bis(anilino)hexafluoropropane, 1,4-bis(anilino)octafluorobutane,1,5-bis(anilino)decafluoropentane, 1,7-bis(anilino)decafluorobutane,2,2-bis[4-(p-aminophenoxy)phenyl]hexafluoropropane,2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane,2,2-bis[4-(2-aminophenoxy)phenyl]hexafluoropropane,2,2-bis[4-(4-aminophenoxy)-3,5-dimethylphenyl]hexafluoropropane,2,2-bis[4-(4-aminophenoxy)-3,5-ditrifluoromethylphenyl]hexafluoropropane,p-bis(4-amino-2-trifluoromethylphenoxy)benzene,4,4′-bis(4-amino-2-trifluoromethylphenoxy)biphenyl,4,4′-bis(4-amino-3-trifluoromethylphenoxy)biphenyl,4,4′-bis(4-amino-2-trifluoromethylphenoxy)diphenylsulfone,4,4′-bis(3-amino-5-trifluoromethylphenoxy)diphenylsulfone and2,2-bis[4-(4-amino-3-trifluoromethylphenoxy)phenyl]hexafluoropropane.

As diamines there may be mentioned siloxanediamines represented by thefollowing general formula (3). In formula (3), R³ represents amonovalent organic group, R⁴ represents a divalent organic group and nrepresents an integer of 1 or greater.

As diisocyanates there may be mentioned diisocyanates obtained byreaction between diamines and phosgene. As specific examples ofisocyanates there may be mentioned aromatic diisocyanates such astolylene diisocyanate, diphenylmethane diisocyanate, naphthalenediisocyanate, diphenylether diisocyanate and phenylene-1,3-diisocyanate.

As tetracarboxylic acids for reaction with diamines, there may be usedones having two pairs of two adjacent carboxyl groups. As specificexamples of tetracarboxylic acids there may be mentioned pyromelliticacid, 2,3,3′,4′-tetracarboxydiphenyl, 3,3′,4,4′-tetracarboxydiphenyl,3,3′,4,4′-tetracarboxydiphenyl ether, 2,3,3′,4′-tetracarboxydiphenylether, 3,3′,4,4′-tetracarboxybenzophenone,2,3,3′,4′-tetracarboxybenzophenone, 2,3,6,7-tetracarboxynaphthalene,1,4,5,7-tetracarboxynaphthalene, 1,2,5,6-tetracarboxynaphthalene,3,3′,4,4′-tetracarboxydiphenylmethane,2,2-bis(3,4-dicarboxyphenyl)propane,2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane,3,3′,4,4′-tetracarboxydiphenylsulfone, 3,4,9,10-tetracarboxyperylene,2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane,2,2-bis[4-(3,4-dicarboxyphenoxy)phenyl]hexafluoropropane,butanetetracarboxylic acid and cyclopentanetetracarboxylic acid. Esters,acid anhydrides and hydrochlorides of these tetracarboxylic acids mayalso be reacted with diamines.

For reaction between the diamine and tetracarboxylic acid or itsderivative, the molar ratio of the diamine or diisocyanate with respectto the tetracarboxylic acid or its derivative is preferably 0.95-1.05.If the ratio is outside of this range for the reaction, the molecularweight of the polyamic acid and the polyimide resin produced therefromwill be reduced tending to result in impaired physical properties of thefilm which may include film brittleness and a poorly maintained filmshape.

The reaction is normally conducted in a solvent such asN-methyl-2-pyrrolidone (NNP), N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), dimethylsulfate, sulfolane, γ-butyrolactone, cresol, phenol, a halogenatedphenol, cyclohexane or dioxane. The reaction temperature is preferably0-200° C.

During the reaction, a modifying compound with a reactive functionalgroup may be added to introduce a crosslinked or ladder structure intothe polyimide resin. Such modifying compounds include, for example,compounds represented by the following general formula (4). Modificationwith such compounds introduces a pyrrolone ring orisoindoloquinazolinedione ring into the polyimide resin.

In formula (4), R⁵ represents a 2+x valent aromatic organic group and Zrepresents —NH₂, —CONH₂, —SO₂NH₂ or —OH, at the ortho position withrespect to the amino group. The letter x represents 1 or 2.

The modifying compound used may be a compound such as an amine, diamine,dicarboxylic acid, tricarboxylic acid or tetracarboxylic acid derivativethat has a polymerizable unsaturated bond. This will result in acrosslinked structured in the polyimide resin. As such compounds theremay be mentioned maleic acid, nadic acid, tetrahydrophthalic acid,ethynylaniline and the like.

The varnish may also contain, in addition to the polyamic acid, apolyimide resin produced by partial reaction of the polyamic acid. Thevarnish may also comprise additional crosslinking components such asepoxy compounds, acrylic compounds, diisocyanate compounds and phenolcompounds, and additives such as fillers, particles, coloring materials,leveling agents, coupling agents and the like. However, if the amount ofsuch additional components is greater than the content of the polyimideresin or its precursor, the properties of the resin film 11 may beimpaired.

The varnish may be coated on the metal foil using a roll coater, commacoater, knife coater, doctor blade, flow coater, sealed coater, diecoater, lip coater or the like. In such cases, the varnish is dischargedfrom a film-forming slit and coated as evenly as possible.

After the varnish has been coated onto the metal foil to form a coatedfilm, the coated film is held for a prescribed time in that state on themetal foil (holding step). The conditions for holding, such as theholding temperature, atmosphere and time, are preferably set so that thecontent of metal elements in the resin film 11 does not significantlyincrease the relative permittivity of the resin film 11, and inconsideration of production cost and equipment investment. Thetemperature, atmosphere and time for the holding step are even morepreferably set in consideration of all of the aforementioned matters ineach of the steps up to the resin film-forming step.

A higher holding temperature and longer holding time may result in ahigher metal element content. Also, a more oxidizing atmosphere for theholding atmosphere may result in a higher metal element content than areducing atmosphere. However, the preferred ranges for the conditionswill probably differ depending on the metal foil 12 material and thevarnish composition. Thus, the conditions are preferably set afterdetermining the correlation between the metal foil 12 material, thevarnish composition and the holding temperature, atmosphere and time,and the metal element content in the resin film 12.

Adjustment of the conditions in the holding step, for example when thetemperature for holding is room temperature and the atmosphere is air inorder to minimize production cost and equipment investment, will involveadjusting the holding time so that the content of metal elements in theresin film 11 is within the preferred range of no greater than 5 wt %.However, adjustment of the conditions is not limited to this method.

After the coated film has been held for the prescribed time in thatstate on the metal foil, it is heated at preferably 100-170° C. (morepreferably 100-160° C.) to remove the solvent in the varnish to aproportion of 1-60 wt % (more preferably 25-45 wt %) of the total inorder to form a resin composition layer (drying step). Here, the heatingmay be accomplished under a reduced pressure atmosphere or under areducing atmosphere as described hereunder. If the proportion of solventafter the drying step is lower than 1 wt %, the resin composition layeror resin film will shrink in subsequent steps, often producing warpingin the obtained flexible laminated body. If the proportion of solventafter the drying step is higher than 60 wt %, the outer appearance maybe impaired due to foaming and the handling property may be reduced dueto excess tack, when the layer is heated to form a resin film.

The conditions for the drying step, such as the temperature, atmosphere,pressure and time for drying and the residual amount of solvent in thevarnish, are preferably set so that the metal element content in theresin film 11 does not significantly increase the relative permittivityof the resin film 11, in consideration of production cost and equipmentinvestment, and also in consideration of minimizing warping of theflexible laminated body, impairment of the outer appearance of the resinfilm due to foaming and reduced handling property due to excess tack.The conditions for the drying step are even more preferably set inconsideration of all of the aforementioned matters in each of the stepsup to the resin film-forming step.

A higher drying temperature and longer drying time may result in ahigher metal element content. Also, a more oxidizing atmosphere for thedrying atmosphere may result in a higher metal element content than areducing atmosphere. However, the preferred ranges for the conditionswill in most cases differ depending on the metal foil 12 material andthe varnish composition. Thus, the conditions are preferably set afterdetermining the correlation between the metal foil 12 material, thevarnish composition and the drying temperature, atmosphere, pressure andtime, and the metal element content in the resin film 12.

Adjustment of the conditions for the drying step may include, inconsideration of minimizing warping of the flexible laminated body,impairment of the outer appearance of the resin film due to foaming andreduced handling property due to excess tack, for example, controllingthe drying temperature to 100-170° C. and the residual solvent in thevarnish to a range of 1-60 wt % of the total, and when the atmosphere isair, controlling the temperature range and residual solvent so that themetal element content in the resin film 11 is no greater than thepreferred limit of 5 wt %. However, adjustment of the conditions is notlimited to this method.

Subsequent heating of the resin composition layer forms a resin film 11containing a polyimide resin (resin film-forming step).

The conditions for the resin film-forming step, such as the temperature,atmosphere and time for formation of the resin film, are preferably setso that the content of metal elements in the resin film 11 does notsignificantly increase the relative permittivity of the resin film 11,and in general consideration of production cost and equipment investmentand of the physical properties of the flexible laminated body orflexible printed circuit board. The conditions for the resinfilm-forming step are even more preferably set in consideration of allof the aforementioned matters in each of the steps up to the resinfilm-forming step.

A lower resin film-forming temperature and longer time may result in ahigher metal element content. Also, a more oxidizing atmosphere for theresin film-forming atmosphere may result in a higher metal elementcontent than a reducing atmosphere. However, the preferred ranges forthe conditions will in most cases differ depending on the metal foil 12material and the composition of the varnish or resin composition layer.Thus, the conditions are preferably set after determining thecorrelation between the metal foil 12 material, the composition of thevarnish or resin composition layer and the resin film-formingtemperature, atmosphere and time, and the metal element content in theresin film 12.

Adjustment of the conditions for the resin film-forming step mayinclude, in consideration of the properties of the flexible laminatedbody or flexible printed circuit board, for example, controlling theatmosphere so that the metal element content in the resin film 11 is nogreater than the preferred limit of 5 wt % when the heating temperatureis controlled to a range of 250-550° C. Specifically, the adjustment ispreferably carried out in the following manner, with the understandingthat adjustment of the conditions is not limited to this method.

Preferably, heating is carried out to 250-550° C. (more preferably250-400° C.) under a reducing atmosphere in the resin film-forming stepto form a polyimide resin-containing resin film 12. The solventremaining in the resin composition layer is removed in the resinfilm-forming step. However, it is sufficient if the solvent isessentially removed, and the flexible laminated sheet or flexibleprinted circuit board may contain a trace amount of residual solvent solong as the properties are not impaired. When the varnish and resincomposition layer contain a polyamic acid, heating in the resinfilm-forming step produces a polyimide resin from the polyamic acid.

A reducing atmosphere is an atmosphere formed from a mixed gas ofessentially an inert gas and a reducing gas. The mixed gas preferablycontains substantially no oxygen, and specifically the mixed gaspreferably has an oxygen concentration of no greater than 5 vol %.Adequate management of the oxygen concentration with an oxygendensitometer in the resin film-forming step is important for quality andsafety.

The inert gas may be helium, neon, argon, nitrogen, or a mixturethereof. Nitrogen gas is preferred among the above from the viewpoint ofconvenient manageability. Hydrogen gas is preferred as a reducing gas.

The reducing atmosphere is most preferably one formed by a mixed gascomposed of nitrogen gas and hydrogen gas at between 0.1 vol % and 4 vol% of the total. If the hydrogen gas concentration is less than 0.1 vol %the effect of the invention will tend to be reduced, and if it isgreater than 4 vol % the lower explosive limit for hydrogen gas may beexceeded. For further improved reliability, the concentration ofhydrogen gas in the mixed gas is more preferably between 0.1 vol % and 1vol %.

Formation of the resin film 11 by heating in a reducing atmosphereprevents oxidation of the polyimide resin. It will also allow control ofmigration of metal elements from the metal foil 12 into the resin film11, so that a resin film with high reliability can be obtained. Anotheradvantage is that coloration of the resin film is prevented andworkability during the process is improved.

Migration of metal elements during the resin film-forming step isbelieved to occur upon exposure to an oxygen-containing atmosphere athigh temperatures of 200° C. and above. Exposure to an oxygen-containingatmosphere at high temperature promotes oxidation of the metal foil 12and may lead to instability of the metals on the surface. Moreover,presumably because of the high acidity of the varnish or resincomposition, the metals on the surface of the metal foil 12 more readilyelute and metal migration takes place. In order to inhibit thisphenomenon, it is important to maintain an oxygen-shielded state in theaforementioned reducing atmosphere.

The flexible laminated sheet of the invention may have a constructiondifferent from the flexible laminated sheet 1 a shown in FIG. 1, such asone obtained by laminating two laminated bodies 10 each comprising ametal foil 12 bonded on one side of a resin film 11, as shown in FIG. 2or 3.

The flexible laminated sheet 1 b shown in FIG. 2 has a constructionwherein two laminated bodies 10 are laminated with their resin films 11bonded together. The flexible laminated sheet 1 b is obtained, forexample, by thermocompression bonding of two laminated bodies 10. Themethod of thermocompression bonding may be a pressing method orlaminating method.

The flexible laminated sheet 1 c shown in FIG. 3 comprises two laminatedbodies 10 and an adhesive layer 15 sandwiched between their resin films11. That is, the flexible laminated sheet 1 c has two laminated bodies10 bonded with an adhesive. The flexible laminated sheet 1 c isobtained, for example, by thermocompression bonding of two laminatedbodies 10 having an adhesive sandwiched between them. The adhesive isnot particularly restricted so long as it is capable of bonding theresin films. For example, a resin composition containing an epoxy resin,acrylic resin, polyamideimide resin, thermoplastic polyimide resin orthe like may be used as the adhesive.

A flexible printed circuit board may be obtained by a method in which aportion of the metal foil on the flexible laminated sheet is removed toform a conductor pattern. FIGS. 4 and 5 are cross-sectional views ofseparate embodiments of a flexible printed circuit board according tothe invention.

The flexible printed circuit board 2 a shown in FIG. 4 is provided witha resin film 11 and a conductor pattern 20 formed on one side of theresin film 11. The conductor pattern 20 is formed by removing portionsof the metal foil 12 on the flexible laminated sheet 1 a and performingpatterning. Patterning of the metal foil 12 may be carried out by amethod such as photolithography. Alternatively, the metal foil 12 may beremoved from the flexible laminated sheet 1 a and electric conductormaterial directly written onto the exposed resin film 11 to form aconductive pattern, in order to obtain the flexible printed circuitboard 2 a.

The flexible printed circuit board 2 b shown in FIG. 5 is provided withan insulating layer comprising two resin films 11, and a conductivepattern 20 formed on both sides of the insulating layer. The flexibleprinted circuit board 2 b can be produced by the same method as theflexible printed circuit board 2 a using the flexible laminated sheet 1b of FIG. 2.

The modes described above are only preferred modes of the invention, andthe invention is not limited to these preferred modes. For example, thesteps in which the conditions are adjusted based on a target for thecontent of metal elements in the resin film are not limited to theholding step, drying step and resin film-forming step. When a holdingstep in which the resin composition layer-formed metal foil is held inthat state is carried out between the coating step and resinfilm-forming step, the temperature, atmosphere and time may be adjustedso that the metal element content is kept within the prescribednumerical range.

EXAMPLES

The present invention will now be explained in greater detail byexamples. However, the invention is not limited to these examples.

The relative permittivities, metal element contents, coefficients oflinear thermal expansion and metal foil peel strengths of the resinfilms in the examples and comparative examples were measured accordingto the following methods.

Relative Permittivity

A rectangular test piece with a width of 2 mm and a length of 80 mm wascut out from the resin film obtained by removing the metal foil of theflexible laminated sheet by etching, and was dried at 105° C. for 30minutes. A stack of 200 dried test pieces was used to measure therelative permittivity at 5 GHz by a cavity resonator method. Measurementof the relative permittivity was accomplished using a “CP511” cavityresonator by Kantoh Electronics Application and Development Inc. and a“E7350A” network analyzer by Agilent Technologies.

Coefficient of Linear Thermal Expansion

A rectangular test piece with a width of 5 mm and a length of 15 mm wascut out from the resin film obtained by removing the metal foil of theflexible laminated sheet by etching. The test piece was used formeasurement of the coefficient of linear thermal expansion in tensilemode. The measurement was performed using a “TMA2940” by TA Instruments,and the coefficient of linear thermal expansion was calculated from theelongation from 50° C. to 250° C.

Metal Element Content

A rectangular test piece with a width of 10 mm and a length of 15 mm wascut out from the resin film obtained by removing the metal foil of theflexible laminated sheet by etching. The metal element content on themain side adjacent to the metal foil of the test piece was measured withan XPS apparatus. Next, argon etching was performed from the main sideto a depth of 0.05 μm (approximately 10 minutes), and the metal elementcontent on the exposed side was measured. This was followed by argonetching for approximately 10 minutes and measurement of the metalelement content 6 times in the same manner, for a total of 8 metalelement content measurements. The metal element content in the resinfilm was then calculated from the measured metal element contents. TheXPS apparatus used was a Model ESCA5400 (trade name of Ulvac-Phi, Inc.).

Metal Foil Peel Strength

The metal foil of the flexible laminated sheet was etched with a 1mm-wide mask to prepare a test piece with a 1 mm-wide metal foil. Theload at which the 1 mm-wide metal foil portion peeled with a peel angleof 90 degrees and a peel rate of 50 mm/min was measured, and the maximumload was recorded as the peel strength.

Synthesis Example

After placing 867.8 g of p-phenylenediamine, 1606.9 g of4,4′-diaminodiphenyl ether and 40 kg of N-methyl-2-pyrrolidone in a 60 Lstainless steel reactor equipped with a thermocouple, stirrer andnitrogen inlet port while circulating approximately 300 mL/min ofnitrogen, the mixture was stirred to dissolve the diamine component. Thesolution was cooled to below 50° C. with a water jacket while slowlyadding 4722.2 g of 3,3′,4,4′-biphenyltetracarboxylic dianhydride topromote polymerization reaction, thereby obtaining a viscous polyamicacid varnish containing polyamic acid and N-methyl-2-pyrrolidone. Inorder to improve the coated film workability, it was cooked for 80° C.until the rotating viscosity of the varnish reached 10 Pa·s.

Coating Example 1

The polyamic acid varnish obtained in the synthesis example was coatedto a thickness of 50 μm on the roughened surface of a copper foil usinga coating machine (comma coater). The copper foil used was a rolledcopper foil with a width of 540 mm and a thickness of 12 μm which hadbeen roughened on one side (“BHY-02B-T”, trade name of Nikko MaterialsCo., Ltd.). After holding it in air at room temperature for a prescribedretention time, a forced-draft drying furnace was used to remove thesolvent from the polyamic acid varnish coated on the copper foil, to aresidual solvent content of 20 wt %, thus forming a resin compositionlayer containing the polyamic acid.

Coating Example 2

A resin composition layer was formed in the same manner as CoatingExample 1, except that the polyamic acid varnish obtained in thesynthesis example was coated to a thickness of 50 μm and the solvent wasremoved from the polyamic acid varnish coated on the copper foil untilthe proportion of solvent was 50 wt %.

Coating Example 3

A resin composition layer was formed in the same manner as CoatingExample 1, except that the polyamic acid varnish obtained in thesynthesis example was coated to a thickness of 50 μm and the solvent wasremoved from the polyamic acid varnish coated on the copper foil untilthe proportion of solvent was 70 wt %.

Examples 1-6, Comparative Examples 1-3

The resin composition layers formed by the methods of Coating Examples1-3 were continuously heated using a hot air circulation oven under theheating conditions shown in Table 1 or 2, to fabricate flexiblelaminated sheets. The relative permittivities, coefficients of linearthermal expansion and metal element contents of the resin films and thepeel strengths of the metal foils in the fabricated flexible laminatedsheets were measured according to the methods described above. Theresults are shown in Tables 1 and 2. In the tables, “0.5% hydrogen”means that a nitrogen/hydrogen mixed gas containing 0.5 vol % hydrogenwas used. The same applies for “2% hydrogen” and “0.1% hydrogen”.

TABLE 1 Example Example Example Example Example Example 1 2 3 4 5 6Resin Coating Coating Coating Coating Coating Coating compositionExample 1 Example 1 Example 1 Example 1 Example 1 Example 2 layerformation Heating Nitrogen/0.5% Nitrogen/0.5% Nitrogen/0.5% Nitrogen/2%Nitrogen/0.1% Nitrogen/0.5% conditions hydrogen hydrogen hydrogenhydrogen hydrogen hydrogen Atmosphere 250-400 250-300 250-550 250-400250-400 250-400 Temperature ° C. Relative 3.1 3.2 3.1 3.3 3.1 3.5permittivity Metal element 3.8 2.2 3.6 4.1 4.5 3.8 content (wt %) Linearthermal 23 25 22 23 22 22 expansion coefficient (ppm/° C.) Copper foil1.4 1.2 1.2 1.3 1.4 1.5 peel strength (kN/m)

TABLE 2 Comparative Comparative Comparative Comparative Example 1Example 2 Example 3 Example 4 Resin Coating Coating Coating Coatingcomposition Example 1 Example 1 Example 1 Example 3 layer formationHeating conditions Atmosphere Nitrogen Air Nitrogen/ Nitrogen/ 0.5% 0.5%hydrogen hydrogen Temperature 250-400 250-400 200 250-400 ° C. Relative3.5 3.6 3.5 Un- permittivity measurable Metal element 8.0 10.0 8.6 Un-content (wt %) measurable Linear thermal 24 27 33 Un- expansionmeasurable coefficient (ppm/° C.) Copper foil 1.5 0.8 0.9 Un- peelstrength measurable (kN/m)

As shown in Table 1, the flexible laminated sheets of the Examplesexhibited satisfactory properties in terms of relative permittivity,coefficient of linear thermal expansion and copper foil peel strength.In contrast, the flexible laminated sheets of Comparative Examples 1-3were inadequate in at least one of these properties. In the case ofComparative Example 4, the obtained copper foil-clad flexible laminatedbody generated significant foaming at the resin film sections and couldnot produce a copper-clad flexible board material with a satisfactoryouter appearance, and therefore evaluation of the relative permittivityand other properties could not be performed.

1. A process for production of a flexible laminated sheet having atleast one laminated body, each laminated body being provided with ametal foil formed on one side of a resin film, the process comprising: acoating step in which a varnish containing a polyamic acid and asolvent, for forming said resin film of each laminated body, is coatedonto the metal foil to form a coated film; a drying step in which thesolvent in the varnish is removed to a proportion of 1-60 wt % of thetotal, to form a resin composition layer from the varnish; and a resinfilm-forming step in which the resin composition layer is heated to250-550° C. under a reducing atmosphere to form said resin film, saidresin film containing a polyimide resin.
 2. A production processaccording to claim 1, wherein the reducing atmosphere is formed by amixed gas composed of nitrogen gas and hydrogen gas at between 0.1 vol %and 4 vol % of the total.
 3. A production process according to claim 1,wherein the solvent is removed by heating the varnish to 100-170° C. inthe drying step.
 4. A production process according to claim 1, whereinthe metal foil is a copper foil.
 5. A production process according toclaim 1, wherein a concentration of the polyamide acid in the varnish is8-40%, and the viscosity of the varnish is 1-40 Pa·s.
 6. A productionprocess according to claim 1, wherein in said drying step, the solventin the varnish is removed to a proportion of 25-45 wt %.
 7. A productionprocess according to claim 1, wherein in the coating step, the varnishis coated in contact with the metal foil.
 8. A production processaccording to claim 1, wherein in the flexible laminated sheet produced,a content of metal elements in the resin film is no greater than 5 wt %.9. A production process according to claim 1, wherein the resinfilm-forming step is performed after the drying step.
 10. A productionprocess according to claim 1, wherein the drying step is performed undera reduced pressure atmosphere or a reducing atmosphere.